In the field of electrostatic image formation using a dry developer, studies have been directed to reduction of toner particle size aiming at achievement of development of a finer electrostatic latent image with high reproducibility. However, mere reduction in toner particle size tends to cause adhesion of the toner onto the background area, called fog. In order to solve this problem, a toner satisfying the relation 3.7-0.11d .ltoreq. or .ltoreq.6.5-0.23d has been proposed, wherein d is a volume average diameter of a toner and or is a residual magnetization, has been proposed as disclosed in JP-A-1-221757 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
According to the above-mentioned technique, the image density is increased to some extent, and fine line reproducibility, gradation, and resistance to fog can be improved. However, as the particle size of a toner is decreased, the magnetic force is increased. As a result, the magnetic cohesive force increases, which is unfavorable for obtaining high definition. Therefore, the technique no more meets the latest strict demands for high image quality such as a high image density o at least 1.4, and preferably around 1.5, as measured with a reflection densitometer, excellent fine line reproducibility, excellent gradation, and freedom from fog. Additionally, the content of a magnetic substance in the toner is increased as compared with conventional toners so as to increase the magnetic force while reducing the size of the toner. However, an increase in magnetic substance content results in an increase in toner specific gravity. It follows that the requisite amount of the toner per copy, that is, a toner consumption will increase, which is against resources saving. Further, a latent image carrier is easily abraded during a cleaning step, tending to have reduced durability. Furthermore, toner fixing properties tend to be deteriorated with an increase of magnetic substance in content.
Hence, improvements have been added to a magnetic substance per se. For example, JP-A-3-155562 teaches a toner having a volume average particle size of not greater than 10 .mu.m in which a hexagonal magnetic substance having a particle size of from 0.1 to 0.3 m is used. Since the toner particles have a moderately uneven surface due to projections of the magnetic substance, the chargeability of the toner is easy to control, and the toner is excellent in environmental stability and durability. However, the toner is still insufficient for meeting such high demands as high reproducibility in developing a very fine latent image of 600 dpi (dot per inch) or 800 dpi, a high image density of about 1.5 as measured with a reflection densitomer, and freedom from fog.
It is now deemed important to develop a magnetic toner while minimizing the magnetic substance content. The confronting problem resides in that mere reduction in magnetic substance content leads to various disadvantages in addition to fog. For example, the quantity of charge would increase more than necessary to cause charge-up phenomena, that is, a reduction in image density and fogging particularly in a low temperature and low humidity environment. Further, the effect of the toner on polishing the surface of a latent image carrier would be lessened, tending to cause image disturbances, such as a smeared image due to contaminants generated by a charger.
JP-A-2-284158 proposes a toner having a chargeability of from --20 to --35 C/g. Although this toner succeeds in inhibiting image density reduction attributed to an increased charge quantity by charge-up phenomena during copying, it is still insufficient for meeting such high demands for image quality as high definition, high reflection density of about 1.5, and freedom from fog, similarly o the toner disclosed in JF-A-3-155562.
As discussed adore, none of the conventional toners satisfies the strict demands o image quality, and they are not always satisfactory in other performance properties.